1. Field of the Invention
The present invention relates generally to piezoelectric resonators and electronic components containing such resonators. More particularly, the invention relates to a piezoelectric resonator which makes maximum use of the mechanical resonance of a piezoelectric member vibrating in the longitudinal vibration mode. The invention also relates to electronic components, such as filters, discriminators, and oscillators, using the above type of piezoelectric resonator.
2. Description of the Related Art
FIG. 15 is a perspective view of a typical conventional piezoelectric resonator. A piezoelectric resonator generally indicated by 1 has a single piezoelectric substrate 2 having, for example, a rectangular planar shape, as viewed from above. The single substrate 2 is polarized in the thickness direction. Electrodes 3 are formed on two opposite major surfaces of the single substrate 2. A signal is input between the electrodes 3 so as to apply an electric field to the single substrate 2 along its thickness, causing the single substrate 2 to vibrate in the longitudinal direction. Also, in FIG. 16. there is shown a piezoelectric resonator 1 in which electrodes 3 are formed on two opposite major surfaces of a single piezoelectric substrate 2 having a square planar shape, as viewed from above. In this resonator 1, as well as the resonator 1 shown in FIG. 15, the single piezoelectric substrate 2 is polarized in the thickness direction. A signal is input between the electrodes 3 of the resonator 1 so as to apply an electric field to the single substrate 2 along its thickness, thereby causing the single substrate 2 to vibrate in the square-type vibration mode.
These piezoelectric resonators 1 of FIGS. 15 and 16 are of an unstiffened type, in which the direction of vibration differs from the direction of polarization and the direction of the electric field. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator, in which each of the direction of vibration, the direction of polarization, and the direction in which an electric field is applied, are the same. Accordingly, an unstiffened resonator has a relatively small frequency difference .DELTA.F between the resonant frequency and the anti-resonant frequency. This leads to a drawback in which a bandwidth in use is narrow when an unstiffened frequency resonator is used as a filter or an oscillator. Thus, flexibility and freedom in designing the resonator characteristics is limited in the above type of piezoelectric resonator and in electronic components containing such a resonator.
The piezoelectric resonator 1 shown in FIG. 15 utilizes the first-order resonance in the longitudinal mode. At the same time, however, the resonator 1 also generates, due to its structure, large spurious resonances in the odd-number-order higher harmonic modes, such as the third-order and fifth-order modes, and in the width mode. Some solutions to suppress or prevent these spurious resonances have been considered, such as polishing, increasing mass, and changing the shape of the electrode. These solutions, however, increase the manufacturing cost.
Additionally, since the piezoelectric substrate has a rectangular planar shape, the thickness of the substrate cannot be reduced due to limitations imposed by a minimum required strength. Accordingly, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be substantially increased. This makes it extremely difficult for achieving impedance matching with an external circuit. Further, to form a ladder filter by connecting a plurality of piezoelectric resonators alternately in series and in parallel, the capacitance ratio of the series resonator to the parallel resonator needs to be large in order to increase attenuation. However, large attenuation cannot be obtained due to limitations in the shape and arrangement of the resonators as described above, resulting in severe limitations on the flexibility of the resonator and component characteristics, such as the frequency difference .DELTA.F and the capacitance.
In the piezoelectric resonator 1 illustrated in FIG. 16, large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are generated. Further, since this type of resonator must have a large size as compared with a piezoelectric resonator using longitudinal vibration in order to obtain the same resonant frequency, it is difficult to reduce the size of the resonator shown in FIG. 16. Also, when a ladder filter is formed by a plurality of piezoelectric resonators, in order to increase the capacitance ratio between the series resonator and the parallel resonator, the resonators connected in series have increased thickness, and electrodes are formed only on part of a piezoelectric substrate to decrease the capacitance. In this case, since the electrodes are only partially formed, not only the capacitance, but also the difference .DELTA.F between the resonant frequency and the anti-resonant frequency, are reduced. The resonators connected in parallel are accordingly required to have a small frequency difference .DELTA.F. As a consequence, the piezoelectricity of the piezoelectric substrate cannot be effectively used, and the pass bandwidth of the filter cannot be increased.
To solve the above problems, a laminated piezoelectric resonator was proposed by the present inventor, for example, in Japanese Patent Application No. 8-122725 which corresponds to Applicants' co-pending U.S. patent application No. Ser. 08/829,880, (Attorney Docket No. 36856.27) for "PIEZOELECTRIC RESONATOR AND ELECTRONIC COMPONENT CONTAINING SAME"; the disclosures of which applications are incorporated herein by reference. This resonator is a stiffened type, in which each of the direction of vibration and the direction in which an electric field is applied are the same. The resonator has more factors that contribute to desirable filter characteristics, such as the width and thickness of the resonator, and the number of laminated layers used in the resonator, as compared with a known type of piezoelectric resonator formed of only a single substrate.
Then, an examination was made whether improved filter characteristics would be obtained by adjusting the above-described factors when this type of laminated piezoelectric resonator is provided in an electronic component, such as a ladder filter. Through this study it was revealed that the frequency difference .DELTA.F is seriously decreased, and spurious resonances are increased, depending on the dimensions, such as the length, width and thickness, of the resonator. This further proves that the determination of the dimensions of the resonator is an important factor.